Mechanisms of sensory processing in the human cortex

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By using magnetoencephalography (MEG), we studied cortical mechanisms of sensory processing in humans on tactile (Inui et al. Cereb Cortex 2004), pain (Inui et al. Cereb Cortex 2006a), auditory (Inui et al. Cereb Cortex 2006b) and visual (Inui and Kakigi J Neurophysiol 2006; Inui et al. Neuroimage 2006) systems. Results showed several common features of processing among these sensory modalities. First, there are several parallel streams. For example, there are at least two streams in auditory processing running posterosuperiorly (from the primary auditory cortex to the belt region and then to the posterior region) and anteriorly (primary auditory cortex-belt-superior temporal gyrus). Second, the time delay between the two sequential activations is about 4 ms, for example, 3.6 ms between areas 3b and 1 and 4.4 ms between areas 1 and 5 for somatosensory processing. Third, early cortical activities exhibit reversals of polarity after 10 ms twice, resulting in a characteristic triphasic time course. Finally, later activities that follow several early activities do not show such a reversal of polarity and long lasting. For example, activities in the planum temporale (auditory), secondary somatosensory cortex (tactile and pain), or limbic structures (pain) belong to this type. Therefore, these activities should differ in function from early activities with the triphasic structure. We postulate that a basic role of early cortical activities is to receive inputs from the thalamus or convergent inputs from the thalamus and/or adjacent multiple cortical areas and to send their signals to the next point, while the later long-lasting activities are related to recognition of the stimuli. Early activities some 10 ms in duration or successive activations with a 4-5 ms time delay seem not appropriate for activities relating to recognition.